Small and ultra-sharp cutting blades with precise geometry control are needed in many fields, e.g. to perform precise surgical cuts in organic tissue and for micro-sample preparation. It is known in the art to form cutting blades by microstructuring a substrate of mono-crystalline silicon.
Currently, dimensions of silicon devices can be controlled accurately and repeatably. Silicon as a crystallographic material and its processing have been well characterized by the semiconductor industry. Crystal axes and planes in silicon are typically described in terms of Miller notation. FIG. 1 illustrates this notation on a cubic lattice with the three fundamental translation vectors a1, a2, and a3, where |a1|=|a2|=|a3|, and angles between a1, a2 and a2, a3 and a3, a1 are all 90 degrees. Some of the important planes are indicated, for example, the plane ABCD is designated (001), the plane ABE is designated (111), and the plane GHIK is designated (110). Planes with negative indices are also described using Miller notation. For example, the plane AED is defined by (1,−1,1) and the plane CDF is defined by (−1,−1,−1). If the lattice points are occupied by identical atoms, then the atom configurations in many of the Miller planes are the same. For example, the planes designated (111), (1,−1,1) and (−1,−1,−1) all belong to the same family of {111} planes.
Indices of lattice plane direction, i.e., of the line normal to the lattice plane, are simply the vector components of the direction resolved along the coordinate axes. Thus the (111) plane has a direction written as [111], and so on. Similar conventions are used to define directions relative to the other planes and families of planes.
The intersection line between planes is described by a vector. For example, in FIG. 1 the (−1,1,1) plane CBE and the (−1,1,1) plane CBF intersect along the line BC which is oriented in the [−1,−1,0] direction. In terms of groups of planes and directions, the {111} planes intersect {100} planes along <110> directions. Simple vector calculations yield the intersection angles between planes. For example, the plane normal vectors [−1,−1,1] and [1,1,1] intersect at an angle of 109.47°.
It should be noted that (hkl) refers to any one of a series of parallel planes, and that [uvw] refers to any one of a series of parallel directions in a cubic crystal. This may be seen by a simple shifting of the origin for the references axes.
Mono-crystalline silicon has a diamond lattice wherein each lattice point is occupied with an identical silicon atom. Here, the {111} planes are the closest spaced among the low-index planes with a separation of 3.135 Angstroms. Many fabrication processes are orientation-sensitive; that is, they depend on the direction in which the crystal slice is cut. Many mechanical and electronic properties of the crystal and its surface are orientation-dependent. Wafer manufacturers typically provide silicon wafers with flats to help identify crystal orientations. Silicon wafers with a {100} surface are provided with a flat along a <110> direction. In this orientation, {111} planes intersect the {100} surface parallel and perpendicular to the wafer flat, at an angle of 54.74 degrees.
The atom density in the principal planes can be shown to be in the ratio {100}:{110}: {111}=1:1.414:1.155. Since atoms in these planes have 2, 1, and 1 dangling bonds respectively, the bond densities are in the ratio 1:0.707:0.577. Crystal dissolution is related to the density of broken bonds; therefore it is slowest in the <111> directions, and will delineate the {111} faces. Chemically selective etches will preferentially etch silicon by exposing {111} planes.
Precise, three-dimensional structures are generally fabricated in an etchable substrate material, particularly in silicon, by anisotropic wet etching of the mono-crystalline silicon substrate. Anisotropic etches cause etching certain crystallographic planes of silicon much more rapidly than others by using some etchants such as mixtures of KOH and water or mixtures of ethylene diamine, pyrocatechol, and water. All of these etchants etch the {111} silicon planes much more slowly than the other low order {100} or {110} planes.
Typical values for the relative etch rates for KOH etchants for the three planes of interest are (111)=1, (100)=100, and (110)=170 as discussed in H. Seidel et al, “Anisotropic Etching of Crystalline Silicon in Alkaline Solutions”, J. Electrochem. Soc., Vol. 137, No. 11, November 1990, pages 3612–3626. These values vary, however, the etch rate ratios remain very large, and account for the tremendous anisotropy seen. It is possible to obtain very low-etched surface roughness for a “mirror-like” finish. For most micromachining and active circuit processing, e.g., MOS devices, silicon wafers with a {100} surface are used.
Anisotropic or “orientation-dependent” etchants thus etch much faster in one direction than in another. KOH, for example, slows down markedly at the {111} planes of silicon, relative to etch rates for other planes. In general, the slowest etching planes are exposed as the etch progresses. It is well-known that etching at “concave” or inside corners in {100} silicon stops at intersecting {111} planes. For example, if an opening in an etch mask forms a rectangle, an anisotropic etchant will etch down exposing {111} planes to form a V-groove with respect to two opposing sides.
However, it is also well-known that “convex” or outside corners are undercut with respect to the etch mask, so that if a window in an etch mask is formed which is more complicated in shape than a rectangle, any convex protrusion will etch back to the “farthest” {111} plane given enough time. As disclosed in U.S. Pat. No. 5,338,400, “MICROMACHINING PROCESS FOR MAKING PERFECT EXTERIOR CORNER IN AN ETCHABLE SUBSTRATE,” the complete disclosure of which is incorporated herein by reference, a number of corner compensation techniques are known that limit or eliminate undercutting of convex corners.